Method and apparatus for heating molten steel utilizing a plasma arc torch

ABSTRACT

A method and apparatus for heating molten steel in a steel making operation is disclosed, and which includes a refractory lined vessel having an open top, a removable cover adapted to overlie the top, and at least one plasma arc torch mounted so as to extend through the cover at an angle of about 60°. The arc gas for the torch includes a substantial percentage of nitrogen, which permits a higher power level as compared to conventionally used argon, and the mass flow rate of the arc gas is maintained at a sufficiently high level to produce a continuous blast of gas which impinges at an angle upon any slag formed on the surface of the molten steel with sufficient force to move the slag and expose the underlying molten steel. As a result, the attachment of the arc to the underlying molten steel is facilitated, and the impingement of the gas contributes to the mixing of the steel to avoid heat stratification.

This application is a division of application Ser. No. 817,907, filedJan. 10, 1986 now U.S. Pat. No. 4,734,551.

The present invention relates to a method and apparatus for heatingmolten steel in a steel making operation, and which employs the use of aplasma arc torch.

In a conventional steel making operation, the iron is extracted from theore in a blast furnace, and the resulting molten iron or "hot metal" isdelivered from the furnace to a mixer, which serves as an accumulator.From the mixer, the hot metal is periodically delivered to the basicoxygen furnace where oxygen is blown through the metal, which raises itstemperature by the exothermic reaction between carbon and the gaseousoxygen, and which also results in the impurities of the hot metal beingoxidized and removed. When heating in the basic oxygen furnace iscompleted, the molten steel is poured into a preheated transfer ladle ata captive argon bubbling (CAB) station, where argon is bubbled upthrough the melt to facilitate mixing and where the final alloying andmetallurgy tests take place. The transfer ladle is a very largerefractory lined vessel, typically having a capacity on the order of 225tons of steel.

Upon the final alloying being completed at the CAB station, the ladle istransported by a crane or the like to a continuous casting station, orto a teem ingot mold, for further processing. At the casting station,the steel is delivered from the ladle to a tundish, which in turndistributes the steel through apertures in its bottom to the continuouscasting molds. A similar procedure is employed to fill the teem ingotmolds with the steel.

A significant problem associated with the above conventional steelmaking process resides in the fact that the continuous casting stationis subject to many operational breakdowns, which may cause it to stopfunctioning. When this happens, the ladles with the hot steel and whichare on their way to the casting station, stand idle and rapidly losetemperature. If the temperature drops below a specified minimumtemperature, the steel is no longer suitable for being processed in thecaster, and the ladle is therefore returned and dumped into the basicoxygen furnace as liquid scrap. The resulting "abort" or "recycle" isquite expensive, since much of the cost of the initial processing ofthis metal is lost. While the problem of aborts is particularlyapplicable to the continuous casting operation, it also sometimes occurswhere the steel is delivered to the teem ingot molds.

In an attempt to alleviate the "abort" problem, several steel mills haveinstalled graphite electrode arc heaters to maintain the temperature ofthe steel in the ladles and possibly add superheat. In addition, severalother steel mills have incorporated a separate abort saving station (orcoupler), which acts as a reheating station between the CAB station andthe caster. These present couplers also use a graphite electrode arc toheat the molten steel. However, these systems are not totallysatisfactory, since the heat from the arc does not readily penetrate themolten steel and thus the heat stratifies within the ladle. Also, thereis a metallurgical problem, in that the carbon from the graphiteelectrodes is released into the molten steel.

It has also been proposed to employ plasma arc torches in steel makingfurnaces of various types, note for example U.S. Pat. Nos. 3,496,280 and3,749,803. Plasma torches of the type previously suggested for this usetypically comprise a housing mounting a tungsten cathode at the rearend, with the forward end including a nozzle or collimator which isdirected toward a pool of molten metal. An electrical arc is then struckbetween the cathode and the molten metal, with the molten metal servingas the anode. Also, an inert gas is forced through the nozzle in avortical pattern, with the gas serving to collimate the arc and whilebeing ionized by the arc and raised to a high temperature. However, suchplasma arc torches have not heretofore been found to be suitable for usein heating the large ladles used in the steel making industry, by reasonof the relatively low power capacity of such torches and the otherproblems noted below.

It is recognized in the plasma technology art that the power deliveredby the torch depends in part upon the particular inert gas employed inthe torch. Thus for example, it is known that argon, which is easy toionize and thus has a relatively low electrical resistance, results in arelatively low operating voltage and power level. Nitrogen, which alsohas been used in plasma arc torches, is more difficult to ionize and hasa higher operating voltage and thus a much higher power level. Hydrogenhas also been proposed for use in plasma arc torches, and hydrogen has astill higher operating voltage and power level.

In spite of the above knowledge, the use of nitrogen and hydrogen insteel heating operations has not been considered feasible. Inparticular, it has been the belief of those skilled in the steel makingart that the use of any nitrogen in the arc gas would be totallyunsatisfactory, since the molten steel would absorb the nitrogen andthereby upset the highly sensitive metallurgical requirements of theresulting steel products. Hydrogen on the other hand is too expensive,and is hazardous to use in a steel making atmosphere. For these reasons,it is believed that all prior attempts to utilize plasma arc torches insteel making operations have employed argon as the arc gas, and theseprior attempts have been largely unsuccessful because of the limitedpower capacity available when argon is employed.

A further difficulty associated with the use of plasma arc torches forheating molten steel, resides in the fact that a layer of slaginherently forms on the surface of the molten steel, and the slag isessentially electrically non-conductive. As a result, it has beendifficult to establish the arc through the slag to the underlying melt.

It is accordingly an object of the present invention to provide a methodand apparatus for heating molten steel which avoids the limitations anddisadvantages of the prior systems as noted above, and which isparticularly adapted for heating molten steel in the large transferladles of the type used in steel making operations and so as to avoid"aborts".

It is a more particular object of the present invention to provide amethod and apparatus for heating, molten steel utilizing a plasma arctorch, with the torch being adapted to be operated at a power levelsufficient to maintain or increase the temperature of the molten steelwhile the steel is held in a transfer ladle in a steel making operation.

It is a further object of the present invention to provide a method andapparatus for heating molten steel utilizing a plasma arc torch, whichdoes not adversely effect the metallurgy of the molten steel, and whichassists in the stirring of the molten steel so as to avoid heatstratification in the ladle.

It is still another object of the present invention to provide a methodand apparatus for heating molten steel utilizing a plasma arc torch, andwhich includes provision for establishing and maintaining the arcthrough any slag formed on the surface of the molten steel.

These and other objects and advantages of the present invention areachieved in the embodiment illustrated herein by the provision of amethod and apparatus which includes a steel heating apparatus whichcomprises a refractory lined vessel having an open top, a removablecover adapted to overlie the top, and at least one plasma arc torchmounted so as to extend through the cover. The torch includes a housingdefining a forward end positioned on the under side of the cover, a rearelectrode mounted within the housing, and gas vortex generating meansfor generating a vortical flow of gas which exits from the forward endof the torch. In operation, the vessel is substantially filled with themolten steel, and the cover is then placed so as to overlie the opentop, with the forward end of the torch disposed in spaced relation abovethe surface of the molten steel in the vessel. The torch is thenoperated to produce a plasma arc having one end thereof attached to therear electrode, and with the arc preferably extending to the moltensteel. A gas which includes a substantial quantity of nitrogen issupplied to the gas vortex generating means such that the vortical flowof gas contains a substantial percentage of nitrogen and such that thetorch may be operated at a relatively high power level. Surprisingly, ithas been found that the presence of substantial quantities of nitrogenin the arc gas, and indeed up to 100% nitrogen, have had no deleteriouseffects on the metallurgy of the resulting steel, and no significantpick-up of the nitrogen has been detected.

As a further aspect of the present invention, a further inert gas, suchas argon, may be concurrently supplied to the vortex generating meanswith the nitrogen, and while the relative percentages of nitrogen andthe other gas are adjustably controlled so as to permit adjustment ofthe power level of the torch. Preferably, during normal operation, thepercentage of nitrogen in the gas supplied to the gas vortex generatingmeans is between about 80 to 100 percent.

As still another aspect of the present invention, the quantity of gassupplied to the vortex generating means is supplied at a relatively highmass flow rate which is sufficient to produce a continuous blast of gasfrom the forward end of the torch. Also, the torch is preferably mountedin the cover at an angle of between about 30° to 70° with respect to thesurface of the cover, and the high gas flow rate acting in conjunctionwith the angular disposition of the torch causes the gas to impinge atan angle upon any slag on the surface of the molten steel, causing theslag to move and expose the underlying molten steel. This not onlyfacilitates the attachment of the arc between the rear electrode and theunderlying exposed molten steel, but also contributes to the mixing ofthe molten steel to thereby reduce heat stratification.

The present invention also involves a novel starting procedures for thetorch, wherein a relatively large quantity of pressurized gas issupplied to the vortex generating means so as to produce an initialblast of gas from the forward end of the torch which impinges at anangle upon any slag on the surface of the molten steel to expose theunderlying molten steel. Concurrently, there is established anelectrical potential between the rear electrode and the molten steel inthe vessel so as to cause the electrical arc to jump between the rearelectrode and the exposed molten steel.

Some of the objects and advantages of the present invention having beenstated, others will become apparent as the description proceeds, whenconsidered in conjunction with the accompanying drawings, in which

FIG. 1 is a schematic illustration of a portion of a steel makingoperation, and which incorporates the features of the present invention;

FIG. 2 is a sectional view of a steel transfer ladle and plasma archtorch in accordance with the present invention; and

FIG. 3 is a partially schematic sectional view of a plasma arc torchsuitable for use with the present invention.

Referring more particularly to the drawings, FIG. 1 illustrates a basicoxygen furnace 10, which is designed to receive molten iron from theblast furnace or an intermediate mixer or accumulator at a temperatureof between about 2100° to 2400° F. In addition, metal scrap and variousferro alloys may be added to achieve the desired metallurgicalcomposition. Oxygen is blown through the metal in the furnace, causingits temperature to rise to about 300° to 3100° F., which results in theimpurities of the metal being oxidized and removed. When heating in thefurnace is complete, the vessel is rotated or "turned down" as shown indashed lines in FIG. 1 to deliver the molten steel 11 (or melt) to apreheated ladle 12 at the captive argon bubbling (CAB) station, wherethe final alloying and metallurgical tests take place. The ladle 12 isgenerally cylindrical and includes a refractory lining 14 and an opentop, and typically has a capacity of about 225 tons of steel. Also, theladle 12 may include a porous plug 16 adjacent the bottom of the ladlefor the introduction of argon or other inert gas which bubbles upthrough the steel and which contributes to the mixing of the moltenmetal during final alloying as further described below.

Upon the final alloying being completed, the ladle 12 is transported bya crane or the like to continuous casting station 20, or to a teem ingotmold station (not shown) for further processing. At the casting station20, the steel is delivered from the ladle 12 to a tundish 22, which inturn distributes the steel through apertures in its bottom to thecontinuous casting mold 24 which casts the ingots 25. Also, a cover 30is placed over the open top of the filled ladle during transport betweenthe oxygen furnace and the casting station to minimize heat loss.

In accordance with the illustrated embodiment of the present invention,two plasma arc torches 32 of like design are mounted in the cover 30 andso as to extend through the cover. When viewed in plan as seen in FIG.3, the torches 32 are mounted approximately midway between the centerand inside periphery of the ladle, on planes that are perpendicular toradial lines. The torches 32 also inclined in the circumferentialdirection at an angle of between about 30° to 70° with respect to thesurface of the cover and the surface of the underlying melt. The mostpreferred angle is about 60°. In addition, each torch 32 is preferablymounted to the cover so as to permit the torch to be adjustably movedalong its longitudinal axis in the direction of the arrows 33 so thatthe spacing between the forward end of the torch and the level of themelt may be adjusted. Further, when again viewed in plan, the twotorches are disposed on opposite sides of the center of the ladle andthey are oriented in opposite directions so as to be adapted to impart acommon circular movement to the molten metal in the ladle, note FIG. 3.

The construction of each torch 32 is generally conventional, and as bestseen in FIG. 2, each torch includes a tubular housing 34 which mounts acup-shaped rear electrode 35, a nozzle 36 or collimator adjacent theforward end of the torch, and a gas vortex forming ring 37 disposed inthe housing coaxially between the rear electrode and nozzle. A gassupply system is provided for supplying pressurized gas to the ring 37such that a helical or vortical flow of gas is formed between the rearelectrode 35 and nozzle 36 and which flows forwardly through the nozzle.The torch also includes a water cooling circulation system, and a DCpower supply for 38 establishing an electrical potential between therear electrode and the molten steel in the vessel, so as to cause anelectrical arc to jump between the rear electrode and the molten steel,with the gas vortex serving to closely collimate the arc. Asillustrated, the rear electrode 35 is the anode of the system, and agraphite electrode 40 extends through the cover and is immersed in thesteel, so that the electrode 35 and molten steel serves as the cathode.Further constructional details of a torch of the described type may beobtained from prior U.S. Pat. No. 4,549,045, the disclosure of which isincorporated by reference.

As shown schematically in FIG. 2, the gas supply system includes asource of nitrogen, and a separate source of argon, together with anadjustable control 42 by which the percentage and amount of each gasdelivered to the gas vortex forming ring 37 may be regulated. Thus it ispossible to operate the torch using only nitrogen, or only argon, or anyrelative amounts of the two gases. By this arrangement, the power levelof the torch may be effectively controlled by adjusting the percentageof nitrogen in the arc gas, with the power level increasing as thepercentage of nitrogen increases.

A torch suitable for use with conventional steel transfer ladles havinga capacity of about 225 tons, may have a power rating of about 3000 kw.Most conventional plasma torches of this size are designed to operatewith a gas flow rate less than about 10 scfm, although higher flow rateshave been previously tested in other specific applications. Inaccordance with the present invention however, the flow rate issignificantly increased beyond the normal flow rates, for example toabout 100 scfm for a 3000 kw torch, which has been found to provideseveral unexpected and significant advantages. Specifically, therelatively large quantity of pressurized gas which is introduced intothe torch produces a continuous blast of gas from the forward end of thetorch which tends to blow through any slag on the surface of the melt soas to expose the melt itself. Further, the high flow rate, inconjunction with the angled orientation of the torch, causes the blastof gas to impinge at an angle on the surface of the melt, which not onlyfacilitates blowing through the slag to expose the melt, but alsocomplements the effect of the argon bubbling to effect stirring of themelt itself, both downwardly as illustrated schematically in FIG. 2, andcircularly as seen in FIG. 3. Thus the tendency of heat stratificationwithin the melt is minimized. Still further, the high gas flow rateserves to closely collimate the arc, and it is believed that a highpercentage of the radiant energy of the arc is absorbed by the gas andtransferred by convection to the melt, rather than being radiated fromthe arc to the walls of the vessel and the cover.

The inside of the cover 30 is preferably lined with arefractory-material 42, which may for example comprise alumina fibers,refractory bricks, or a combination thereof. In addition, the covermounts the graphite electrode 40 as noted above, and the cover alsoincludes an exhaust port 44 to exhaust the gases from the interior ofthe ladle.

As noted above, it is common for slag to form on the surface of the meltas indicated at 11' in FIG. 2, and the presence of the slag renders thestarting of the arc difficult, since the slag is relativelynon-conductive. In accordance with a further aspect of the presentinvention, a starting procedure may be employed wherein the torch isinitially held with its forward end a distance of about 18 inches abovethe slag, and the torch is initially started in a non-transfer modewherein the arc extends between the rear electrode 35 and nozzle 36. Ablast of arc gas is then introduced into the torch which results in thegas pushing the slag 11' to one side to expose the underlying melt,while also carrying the arc directly to the exposed melt. The flow rateof the gas may then be reduced to normal operating levels as describedabove.

In the drawings and specification, there has been set forth a preferredembodiment of the invention, and although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation.

That which is claimed is:
 1. An apparatus for heating a molten metalsuch as steel and comprisinga vessel adapted to support molten metalsuch as steel and including an open top, at least two plasma arc torchesmounted above said vessel, each said torch including a housing defininga forward end positioned adjacent the molten metal supported in saidvessel, a rear electrode mounted within the housing, a tubularcollimator mounted within said housing forwardly of and in coaxialalignment with said rear electrode, power supply means for generating anarc which extends from said rear electrode through said collimator andto the molten metal supported in said vessel, and vortex generatingmeans for generating a vortical flow of gas at a location between saidrear electrode and collimator and so that a vortical flow of gas exitsforwardly through said collimator and substantially surrounds andcollimates said arc, and with each of said at least two plasma torchesbeing mounted approximately midway between the center and the insideperiphery of the vessel and at circumferentially spaced locations withrespect to the center of the vessel and also being mounted along a planethat is perpendicular to a radial line from the center of the vessel andat an angle of between about 30° to 70° with respect to the surface ofthe molten metal in the vessel, and with said torches being oriented ina common circumferential direction about the center of the vessel andsuch that the mass flow provided by said vortex generating means of saidtorches impinges at an angle upon the surface of the molten metal insaid vessel and so as to impart a common circular movement to the moltenmetal in the vessel.
 2. The apparatus as defined in claim 1 wherein saidpower supply means comprises an electrode mounted in said vessel so asto be adapted to contact and ground the molten metal, and a directcurrent power supply, with the anode thereof connected to the rearelectrode of said torch and the cathode thereof connected to saidelectrode.
 3. The apparatus as defined in claim 2 wherein said vortexgenerating means includes means for supplying a gas comprising at leastabout 80% nitrogen.
 4. The apparatus as defined in claim 2 wherein saidvortex generating means includes means for supplying nitrogen andanother inert gas to said vortex generating means, and control means bywhich the relative percentages of nitrogen and the other inert gas maybe adjusted.
 5. The apparatus as defined in claim 1 wherein saidapparatus further comprises a removable cover adapted to overlie saidopen top, and with said two plasma torches extending through said coverwith the forward ends thereof being positioned on the under side of saidcover.
 6. The apparatus as defined in claim 5 wherein said power supplymeans includes an electrode mounted to extend through said cover so asto be adapted to contact and ground the molten metal.
 7. The apparatusas defined in claim 5 wherein said vessel is refractory lined and has agenerally cylindrical outline.
 8. The apparatus as defined in claim 5wherein said cover further includes an exhaust gas port whereby thegases introduced into the vessel by said torch may be exhausted.
 9. Theapparatus as defined in claim 5 wherein the surface of said cover whichis adapted to directly overlie the open top of said vessel includes arefractory lining.